I have a PO box also and have trouble with this. What I had been doing was street address on first line, #(my box number) on the second line. This seemed to work most of the time, but I know that some of my mail was returned, I guess depending on who sorted the mail. Now I fill it out with street address first line, box (my box number) on the second line. I believe this works better but I guess I could be disqualified by a finicky JA. I have been called before and when giving them the address told they did not accept po boxes, gave them my street address and they mailed it and I still received it in my PO box. I guess an advantage of living in a smaller town.
I would be fine with them not accepting PO boxes if they would always use UPS or Fedex, but a lot of them ban PO boxes then mail the prizes. Some of us have no choice but to use PO boxes. I guess it happens, but how many people would really have multiple boxes to get extra entries? I barely have time to enter what I am entering now, much less entering them multiple times.